I extracted your theme and found "GdmGreeterTheme.desktop" and "mohegan.xml", both of with are used by GDM2. So my conclusion is that this is in fact a GDM2 theme.

If this was indeed a GDM3 theme, how would you install it? (There is lots of postings on howto change the gdm background or the _gtk_ theme, but I have found nothing on howto install a gdm2 theme with gdm3.)
- May 15 2011